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Capability Studies - Fewest Number of Parts for a Capability Study


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  Post Number #1  
Old 16th February 2000, 10:06 AM
ivo

 
 
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Question Capability Studies - Fewest Number of Parts for a Capability Study

For my traineeship, I have to improve a production process. I have done an FMEA and now I want to do a capability study. But at school I learned that the least amount of products you should produce (to test) is 50.
But the products that I have to produce are about 13 meters long and 100 kg. Is there a way to reduce the number of products? Please let me know..........

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  Post Number #2  
Old 16th February 2000, 03:55 PM
Mike525

 
 
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ivo:

I'm not a statisticion, but I believe the fewest number of parts one can use for a capability study, and still be statistically correct, is 31 - has something to do with the distribution under a normal curve based on the Standard Normal (z) Table and Sample (t) Table. Two good books to reference are "Introduction to Statistical Quality Control" 2nd ed. by Douglas Montgomery, and "Understanding Statistical Process Control" 2nd ed., by Donald Wheeler and David Chambers. Hope this helps.
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  Post Number #3  
Old 18th February 2000, 04:19 AM
Qualiman

 
 
Total Posts: 61
Lightbulb

ivo:

As a normal rule you cannot make a capability study unless you have already demonstrated the stability of your process, and you need at least 30 data if you use average lectures and more than 100 for individual moving range charts.
However,(I declare : I am not an expert in this field) you can run some short time period (or preliminary)studies to have an idea of your process capability using let us say.. 7 lectures, but in this case you must close your gap and instead of using 3 sigma you have to use 4, I think to "compensate" of using few points.

What other folks think ?

Qualiman
  Post Number #4  
Old 27th February 2000, 08:11 PM
Batman

 
 
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Screw

One should always plan the attack before going into the field. If one is charged with improving a process, one would plan the attack, which would include the "after" sampling, along with the comparison to the "before" stuff.

That said, in this instance, based on the description of the product there are few parts made per day, them how about just adding the new data onto the old in a run chart of some sort. I assume that there was historical data that suggested the need for improvement, so take some "after" parts (seperate enough from the "before" parts - maybe a "purge" of the process) and add the sample data to a run chart of the prior data - Average and moving range, for example. If the large size is an indication of being able to run only a few per hour or day, then I think only a few could be necessary. The improvement (or lack thereof) should be obvious. IF there is an improvement, continue to run more parts, enough to see expected variation over some time, then calculate capability.

I think the first step is to determine if the change was a real improvement.
  Post Number #5  
Old 20th June 2006, 12:29 PM
Marc's Avatar
Marc

 
 
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Re: Capability Studies - Fewest Number of Parts for a Capability Study

Quote:
In Reply to Parent Post by ivo

For my traineeship, I have to improve a production process. I have done an FMEA and now I want to do a capability study. But at school I learned that the least amount of products you should produce (to test) is 50.
But the products that I have to produce are about 13 meters long and 100 kg. Is there a way to reduce the number of products? Please let me know..........
Can anyone clarify the minimum number of parts required for a capability study and the reasoning behind the number?

Anyone know where Mike525 might have gotten the number 31?
  Post Number #6  
Old 20th June 2006, 12:42 PM
Jim Wynne's Avatar
Jim Wynne

 
 
Total Posts: 14,161
Re: Capability Studies - Fewest Number of Parts for a Capability Study

Quote:
In Reply to Parent Post by Marc

Can anyone clarify the minimum number of parts required for a capability study and the reasoning behind the number?

Anyone know where Mike525 might have gotten the number 31?
Generally speaking, in a normally distributed population, the sample and population standard deviations should be nearly identical after ~30 pieces. Generally. There are some problems (among others) with the idea of so-called 30-piece capability studies:
  • If the sampling isn't really random (i.e., each individual has an equal chance of being selected each time), all bets are off.
  • Unless the chronological order of production is preserved in the samples, an important and oft-overlookes component of capability study is lost.
  Post Number #7  
Old 20th June 2006, 01:34 PM
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jrubio

 
 
Total Posts: 547
Re: Capability Studies - Fewest Number of Parts for a Capability Study

Here is what I think, if I am wrong I will be pleased to know it.


-Ppk the procces is cronic none stable. (Minimum 100 parts)
-Cpk the process is stable. (MSA third edition page 7)

---------------------------------

-Preliminary.

Short term.

According to MSA Do not included the effect of people, raw material, method, machine, gages, enviroment).


For new process or revised proccess

3-5 parts > 20 subgroup = 100 parts (According to Ford Design Institute. Page 5 Control Process Course)

and According to PPAP Thied Edition (Sorry but I am not in Automobile Arena).

Need to produce a minimum of 300 parts produced minimum 1 hour to 8 hours.( Page 2) for these 300 parts need to select (according to PPAP thierd Editiion Page 6) Minimum 25 subgrops with total amount of 100 readings of a significant production batch. These samples could be changed to long term if Customer request.

Long term.


According to PPAP

included the effect of people, raw material, method, machine, gages, enviroment). Minimum 100 parts splited during some change in our process.


Therefore with that info I consider that minimum 100 parts.


Therefore 100 parts is the number minimum to calculate the capability.
  Post Number #8  
Old 20th June 2006, 01:38 PM
Jim Wynne's Avatar
Jim Wynne

 
 
Total Posts: 14,161
Re: Capability Studies - Fewest Number of Parts for a Capability Study

Quote:
In Reply to Parent Post by jrubio

Therefore 100 parts is the number minimum to calculate the capability.
You are correct with regard to AIAG requirements, but I think the question pertains to a statistical justification for a minimum number.
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